<div class="htmlview paragraph">Since many electric and electronic systems are continuously added in a vehicle to meet various regulations and customer demands over the last decade, the demand on the electric power have been substantially increased. Furthermore the idle time fraction during the vehicle traveling has been increased due to the heavy urban traffic condition. The electric power system of the modern vehicle has to supply enough electrical energy to numerous electrical and electronic systems.</div> <div class="htmlview paragraph">A detailed understanding of the characteristics of the electric power system, electrical load demands, and driving environment such as road, season, and vehicle weight are required when the capacities of generator and battery are determined for a vehicle. In order to avoid an over or under design problem of the electric power system, a simulation program for electric power estimation is adequate. In order to estimate precisely, it is necessary to develop relatively simple and accurate electrically equivalent models of major systems.</div> <div class="htmlview paragraph">In this study, vehicle electric power system, which is mainly composed of generator and battery, is modeled and evaluated. Among various proposed battery models, two types of battery model are compared in terms of accuracy and easy-of-use. Two models are distinguished by the consideration of in-rush current at the beginning of charging and discharging. As an alternator model, variable terminal voltage model is newly proposed, and also it is compared to constant terminal voltage model. Based on the major component model, a simulation algorithm is developed and used to perform a case study. Compared with the real data from the vehicle, the simulation results of energy generation and consumption are comparable.</div>